1. Field of the Invention
This application generally relates to electrochemical capacitors, more particularly to fibril composite electrodes for electrochemical capacitors.
2. Description of the Related Art
Several publications are referenced in this application. These references describe the state of the art to which this invention pertains, and are incorporated herein by reference.
Electrochemical capacitors (ECs) are gaining acceptance in the electronics industry as system designers become familiar with their attributes and benefits. Compared with conventional capacitors, ECs have extremely high capacitance values, limited frequency response, high equivalent series resistance (ESR) which is directly related to electrode thickness and inversely proportional to the cross sectional area of the electrode, voltage-dependent capacitance, and voltage-dependent self-discharge rate. ECs were originally developed to provide large bursts of driving energy for orbital lasers. In complementary metal oxide semiconductor (CMOS) memory backup applications, for instance, a one-Farad EC having a volume of only one-half cubic inch can replace nickel-cadmium or lithium batteries and provide backup power for months. And in electric vehicle applications, large ECs can "load-level" the power on the battery system and thereby increase battery life and extend vehicle range.
Capacitors store energy in the electric field between two oppositely charged parallel plates, which are separated by an insulator. The amount of energy a capacitor can store increases as the area of conducting plates increases, the distance between the plates decreases, and the dielectric constant (the ability to store charge between the plates) of the insulating material increases.
Electrochemical capacitors can generally be divided into two subcategories: double layer capacitors in which the capacitance at the electrode/electrolyte interface can be modeled as two parallel sheets of charge; and pseudocapacitor devices in which charge transfer between the electrode and the electrolyte occurs over a wide potential range. These charge transfers are believed to be the result of primary, secondary, and tertiary oxidation/reduction reactions between the electrode and the electrolyte.
There are generally two kind of pseudocapacitor materials: metal oxides, (i.e., RuO.sub.2, IrO.sub.2, and CoO.sub.2) and redox conductive polymers (i.e., polyaniline, polypyrrole, and polythiophene). Pseudocapacitors suffer from high material cost and low cell voltage. Metal oxide capacitors are very expensive as many of the preferred metals, such as Ru and Ir, are very costly. Redox polymers have relatively high energy storage capacity, low cost and long cycle life. However, these conductive polymers have a narrow working voltage in proton conducting electrolytes.
The high volumetric capacitance density of an EC (10 to 100 times greater than conventional capacitors) derives from using porous electrodes to create a large effective "plate area" and from storing energy in the diffuse double layer. This double layer, created naturally at a solid-electrolyte interface when voltage is imposed, has a thickness of only about 1 nm, thus forming an extremely small effective "plate separation". In some ECs, stored energy is substantially augmented by so-called "pseudocapacitance" effects, occurring again at the solid-electrolyte interface. Double layer capacitors are commonly of the order of 16-40 .mu.F cm.sup.-2 while pseudocapacitors associated with EC systems are commonly 10-100 .mu.F cm.sup.-2.
The double layer capacitor is based on a high surface area electrode material, such as activated carbon, immersed in an electrolyte. A polarized double layer is formed at each electrode providing double-layer capacitance. The carbon provides a high surface area, A, and the effective d is reduced to an atomic scale, thus providing a high capacitance.
Although the energy storage capability of the double layer was recognized more than 100 years ago, it took the development of low-current-draw volatile computer memories to create a market for ECs.
ECs are distinguishable from traditional electrolytic capacitors which store energy by charge separation across a thin insulating oxide film that is often formed by a controlled electrolytic oxidation process at an appropriate metal.
Conventional electrochemical energy storage is achieved in a galvanic cell or a battery of such cells. The energy corresponds to the charge associated with chemical redox changes that can occur in the battery on discharge, multiplied by the voltage difference between the electrodes of the cell. The discharge process involves a net chemical reaction in the cell associated with passage of a certain number of electrons or faradays per mole of reactants.
If an electrochemical reaction, such as a redox process, should occur at or near the electrode, the capacitance may be further increased. This increased capacitance is sometimes termed "pseudocapacitance" and the resulting device, while properly an electrochemical capacitor, is informally called a pseudocapacitor. An electrochemical capacitor will have a different cyclic voltammogram than a pure double-layer capacitor, the pseudocapacitance revealing a Faradaic signature.
Redox systems, especially of RuO.sub.2.xH.sub.2 O, for electrochemical capacitors have been demonstrated (Zheng, Z. P. and Jow, T. R., "A new charge storage mechanism for Electrochemical Capacitors", J. Electrochem. Soc., 142, L6 (1995)), but high cost and limited cycle life are continuing impediments to commercial use of such materials. The greater the Faradaic component of the capacitance, the more the discharge curves and life approach those of a battery rather than those of a capacitor. On the other hand, the specific goals of obtaining high power output suitable for electric vehicle (EV) applications cannot be met by a pure double layer capacitor using known or proposed electrode materials (Eisenmann, E. T., "Design Rules and Reality Check for Carbon-Based Ultracapacitors", SAND95-0671.cndot.UC-400 April 1995).
ECs do not approach the energy density of batteries. For a given applied voltage, capacitatively storage energy associated with a given charge is half that storable in a corresponding battery system for passage of the same charge. This difference is due to the fact that in an ideal battery reaction, involving two-phase systems, charge can be accumulated at constant potential while, for a capacitor, charge must be passed into the capacitor where voltage and charge is being continuously built up. This is why energy storage by a capacitor is half that for the same charge and voltage in battery energy storage under otherwise identical and ideal conditions.
Nevertheless, ECs are extremely attractive power sources. Compared with batteries, they require no maintenance, offer much higher cycle-life, require a very simple charging circuit, experience no "memory effect", and are generally much safer. Physical rather than chemical energy storage is the key reason for their safe operation and extraordinarily high cycle-life. Perhaps most importantly, capacitors offer higher power density than batteries.
However, presently available EC products are limited in size and power performance, due primarily to their memory backup use. They have capacitance values of up to a few Farads, an equivalent series resistance (ESR) of one to fifty ohms, and a working voltage of 3 to 11 V.
Until recently, ECs suitable for high-power applications have been unavailable. But interest in automotive starting, lighting and ignition (SLI) applications, as well as in electric vehicle (EV) load-leveling, has stimulated product development activities for such high-power devices. The goal is to develop products that can be efficiently charged and then discharged in the time specified for these high-rate applications.
Severe demands are placed on the energy storage system used in an EV. The system must store sufficient energy to provide an acceptable driving range. It must have adequate power to provide acceptable driving performance, notably acceleration rate. In addition, the system must be durable to give years of reliable operation. And finally, the system must be affordable. These four requirements are often in conflict for candidate energy storage technologies. This situation creates significant challenges to developers of EV energy storage systems.
A capacitor offers significant advantages to the EV energy storage system. But to be useful, it must store about 400 Wh of energy, be able to deliver about 40 kW of power for about 10 seconds, provide high cycle-life (&gt;100,000 cycles), and meet specified volume, weight and cost constraints.
Electrochemical capacitors, sometimes called ultracapacitors, or supercapacitors, are of interest in hybrid electric vehicles where they can supplement a battery used in electric cars to provide bursts of power needed for rapid acceleration, the biggest technical hurdle to making battery-powered cars commercially viable. A battery would still be used for cruising, but capacitors (because they release energy much more quickly than batteries) would kick in whenever the car needs to accelerate for merging, passing, emergency maneuvers, and hill climbing. To be cost and weight effective compared to additional battery capacity they must combine adequate specific energy and specific power with long cycle life and meet cost targets, as well.
The energy stored in a charged capacitor can be continuously increased in proportion to the increase of the voltage, limited only by electrical breakdown of the dielectric. The maximum available stored energy, for a given chemical species, is determined by the quantity of electroactive materials, their standard electrode potentials and their equivalent weights, and the power by the reversibility of the electrochemical changes that take place over discharge together with the electrical resistivity of the materials and external circuitry.
Experience with carbon electrode electrochemical capacitors shows that geometrical capacitance calculated from the measured surface area and the width of the dipole layer is not routinely achieved. In fact, for very high surface area carbons, typically only about ten percent of the "theoretical" capacitance seems to be found.
This disappointing performance is related to the presence of micropores and ascribed to wetting deficiencies and/or the inability of a double layer to form successfully in pores in which the oppositely charged surfaces are less than about 20 .ANG. apart. In activated carbons, depending on the source of the carbon and the heat treatment temperature, a surprising amount of surface can be in the form of such micropores (Byrne, J. F. and Marsh, H., "Introductory Overview" in Patrick, J. W., Porosity in Carbons: Characterization and Applications, Halsted, 1995).
The performance characteristics of electrochemical capacitors are fundamentally determined by the structural and electrochemical properties of electrodes. Various materials including doped conducting polymer, metal oxides, metal nitrides, and carbon in various forms have been studied for use as electrode materials.
Several methods are known in the art for increasing the amount of energy stored in an electrochemical capacitor. One such method is to increase the surface area of the active electrode. High surface area electrodes result in an increase in storage capacitance and thus increased stored energy. Another approach for increasing stored energy involves using different types of material for fabricating the capacitor's electrodes. Carbon electrodes are used in most commercial capacitors, while precious metal oxide electrodes are used in a the capacitors known as pseudocapacitors.
Electrochemical capacitors containing electrodes fabricated from more than one material (two-component electrodes) are described in a number of references.
U.S. Pat. No. 4,862,328 to Morimoto et al. describes a polarizable electrode for a coin-shaped double layer capacitor composed of a structure of fluorine-containing polymer resin with a fine carbon powder incorporated therein. The structure includes fine nodes of resin connected by fine fibers of the resin. The carbon powder is contained in the nodes. The fluorine-containing polymer resin is about 5 to 30% by weight relative to the fine carbon powder. A sealing material is interposed in the capacitor.
U.S. Pat. No. 5,079,674 to Malaspina provides for an electrode used in supercapacitors composed of two active electrodes bonded to opposite sides of a dielectric separator. The active electrodes consist of metal oxides, chlorides, bromides, sulfates, nitrates, sulfides, hydrides, nitrides, phosphides or selenides coated onto porous carbon particles. The coated particles are bound together in a matrix of a fluorocarbon resin.
U.S. Pat. No. 5,136,473 to Tsuchiya et al. relates to an electric double layer capacitor having at least two polarized electrodes, a separator interposed between the electrodes, and a casing in which the electrodes, separator, and electrolyte are accommodated. The polarized electrodes are composed of two powders of joined minute active carbon particles, the particle of each powder having different diameters.
In U.S. Pat. No. 5,369,546 to Saito et al., the electric double layer capacitor is characterized in that composite materials of activated carbon/polyacene are composed on conductive layers formed on electrical insulating ceramic substrates and a couple of these polarizable electrodes are arranged as facing each other through a separator.
U.S. Pat. No. 5,501,922 to Li et al. relates to a modified carbon electrode for use in an energy storage device made from an activated carbon support having adsorbed thereon a protonated polymer, the polymer having adsorbed therein a polyoxometalate.
U.S. Pat. No. 5,429,893 to Thomas describes an electrochemical capacitor comprising a first electrode fabricated of a carbon-based material, a second electrode fabricated of an inorganic redox material such as Ru, Rh, Pd, Os, Ir, Co, Ni, Mn, Fe, Pt, and alloys and oxides thereof and an electrolyte disposed between the first and second electrodes.
Similarly, U.S. Pat. No. 5,538,813 to Li covers an electrochemical storage device fabricated from two opposing asymmetric electrode assemblies and a solid polymer electrolyte. The first electrode consists of a conducting polymer selected from polyaniline, polypyrrole, polythiophene, polychlorophenylthiophene, polyfluorophenolthiophene and n or p-doped conducting polymer. The second electrode is fabricated from Al, Fe, In, Mn, Mg, Sb, Mo, Cr, Ni, N, V, An, Ru, Ir, Co, Zn, Sn, Bi, Cd, Pd, Ag or alloys or oxides thereof. A polymer electrolyte is dispersed between the electrodes.
U.S. Pat. No. 5,55 7,497 to Ivanov et al. relates to a capacitor comprising an electrolyte, at least one pair of electrolyte-impregnated electrodes, a separator, at least one pair of current collectors, and an uncompressed gasket. The electrolyte-impregnated electrodes are composed of various forms of carbon particles in combination with porous elastic dielectrics and polymer binders.
U.S. Pat. No. 5,581,438 to Halliop describes a double layer capacitor having a housing, a porous separator, an electrolyte, a conductor and electrodes. The electrodes are formed from a current collector positioned against a non-woven web of non-activated carbon fibers impregnated with carbon particles and positioned on either side of a porous layer within a container including a suitable electrolyte.
In order to achieve improved power performance over earlier capacitor devices, NEC developed an activated carbon/carbon composite electrode with a sulfuric acid electrolyte to be used in a supercapacitor. The electrodes are formed from phenol resin, activated carbon powder and PMMA. The phenol resin was used as a binder.
Merryman (at al. of Auburn University also designed a two component electrode. The double layer capacitors are constructed using a composite carbon/metal electrode structure. Large surface area carbon fibers are blended with nickel fibers and a cellulose binder. This mixture is then converted to a paper sheet. A thin foil backing plate is sandwiched between two pieces of the composite paper material. With the nickel fibers sinter-bonded to each other, a conducting path which does not require pressure to achieve low ESR values is formed throughout the carbon bed.
The two-component electrodes described in these patents and references provide improved electric capacity and/or mechanical properties to the electrochemical capacitor as compared to the prior art electrodes. However, many of these electrodes require the presence of a binder or sealer material (in addition to the electrically conductive materials) to hold the electrode components in the desired shape or in the proper orientation. Further, they are not able to provide sufficient electrical capacitance to be used in many high energy applications and only work with certain electrolytes. Moreover, many of the references describe asymmetric two-component electrodes in which the two components are not combined but are present in different electrodes.
Accordingly, there exists a need to provide novel electrochemical capacitors exhibiting greater capacitance using composite electrodes having high accessible surface area, high porosity and reduced or no micropores and being free of the limitations inherent in prior art systems. There also exists the need to provide for a method of producing uniform, symmetrical electrodes which can be used in capacitors to achieve higher operating voltage levels as well as sealing of the completed electrode. Such electrochemical capacitors should have high ionic conductivity, provide high power and high energy, and be fabricated from relatively environmentally benign materials. In addition to high conductivity, it is important that the composite electrodes exhibit high utilization efficiency of expensive active materials, have high structural as well as chemical stability and have improved processibility. Moreover, fabrication of such composite electrodes should be simple, inexpensive, and readily repeatable.
This invention provides fibril composite electrodes for electrochemical capacitors that achieves these results and which overcomes the problems inherent in the prior art.